This invention relates to a method for producing a variable density article by compacting a plurality of powders having different compositions and/or particle sizes, and in specific amounts, followed by sintering at conditions which result in the final article each section of which has a characteristic density. More particularly, it relates to a method in which the powder is metal powder.
It is often necessary and desirable to produce articles having sections of varying density.
One such application of varying density articles is in the specialty lamp industry in which arc and flash lamps are made in which the electrodes have multi-piece construction. For example, the tip of the electrode is about 80%.+-.10% dense tungsten, impregnated with various emitter compounds. The shank of the electrode is dense tungsten (from about 94% to about 100% of the theoretical density) which acts as a heat sink plus a current conductor. The tip and shank are brazed together.
This assembly brazing is difficult and costly to accomplish. If brazing is done before the tip is impregnated, some of the braze is sucked up into the porous tip. This prevents impregnation of the desirable compounds. If brazing is done after impregnation, adherence and contamination problems occur.
An alternate method of making these electrodes, in particular cathodes, is to use tungsten having a density of about 80% of the theoretical density for the entire electrode. The electrodes are expensive and conduct neither heat nor electricity well.